To warn of slippery roads or other hazardous roadway states, modern motor vehicles typically have external temperature sensors having a corresponding display as standard equipment, in order to give the driver or vehicle occupants a warning, for example, if the external temperature sinks below +3° C., for example. Therefore, the driver is to be warned before the occurrence of possible slippery ice or winter road conditions as a precaution. Such warning messages are of a general nature, however, and are independent of the actual roadway state, so that they are only perceived by the driver and/or a vehicle occupant as marginal information, if at all. In practice, such temperature messages are frequently ignored.
In order to obtain information about the roadway surface actually located under the motor vehicle, light-based optical surface sensors or mechanical or acoustic sensors are known. However, these share the feature of only being able to provide satisfactory results when the vehicle moves in relation to the surface.
A method and a device for outputting a warning signal for the vehicle driver, in order to warn about a slippery underlying surface, is known from WO 2002/081250 A1. The triggering of the warning is based on slip values ascertained on a rotating wheel. This system can only be used when the vehicle is moving.
However, many accidents or hazardous situations result when the vehicle is stopped and occupants get out, or the vehicle and optionally trailers, respectively, are to be parked.